It has been known that after an inorganic coagulant is added to raw water (water to be treated), such as industrial wastewater or sewage water, solid-liquid separation is performed by simultaneously adding an anionic polymer flocculant and a cationic polymer flocculant (comparative examples 6 and 10 of Patent Literature 1).
Patent Literature 2 discloses that after a flocculated product is generated by addition of an inorganic coagulant to raw water, such as city water, sewage water, or industrial waste water an organic polymer flocculant is added, and granular substances are then settled by performing stirring granulation.
Patent Literature 3 discloses that in a drainage water treatment or a water treatment, after a flocculated product is generated by addition of an inorganic coagulant to raw water, an organic polymer flocculant is added thereto, and flocculation/liquid-solid separation is then performed by revolution in a liquid cyclone.
Patent Literature 4 discloses that after an anionic polymer flocculant is added to a suspension liquid containing metal hydroxides, such as drainage water discharged from a plating plant, a cationic polymer flocculant is added, and subsequently, solid-liquid separation is performed by stirring granulation. Since a shearing force is applied to a flocculated product by stirring thereof, mechanical dehydration of flocs is promoted, and as a result, flocculates (pellets) having a high density are formed. Since the pellets thus granulated have a high settling rate than that of normal flocs, the surface loading can be increased as compared to that obtained by a general sludge blanket type flocculating and settling apparatus, and hence, the size of the apparatus can be made compact.
Patent Literature 5 has disclosed a method for treating heavy metal-containing drainage water in which heavy metal-containing water is neutralized and is simultaneously processed by a flocculating treatment by addition of a polymer.
Patent Literature 6 has disclosed a method for treating fluorine-containing water and an apparatus therefor, in each of which after insoluble iron phosphate is generated by a reaction between fluorine-containing water and iron, solid-liquid separation is performed by addition of a polyacrylamide-based polymer flocculant.